Imaging systems are ubiquitous in the modern world, primarily due to progressive improvements in digital imaging technology, allowing high quality at low cost. The cost of image sensors has been reduced drastically over the past few decades, enabling very high quality sensors in even such relatively inexpensive consumer products as smart phones.
Although there has been great progress in reducing the cost of image sensors, especially for use in the visible band of the electromagnetic spectrum (i.e. the band over which human vision functions), the progress in reducing the weight and cost of the optical components that form the image on the image sensor has been less impressive. In many imaging systems, both cost and weight are dominated by the optics, with the image sensor and associated electronics contributing a relatively small fraction of the overall system cost.
In many imaging systems, the portion of the optics that dominates the cost of the optics, and may even dominate the overall cost of the system, is the first optic with optical power encountered by electromagnetic radiation incident on the system. Here an optic having “power” is one that causes convergence of an incident beam. If this first optic is a refractive lens, it is generally referred to as the objective lens. If this first optic is a curved reflective mirror, it generally referred to as the primary mirror.
One of the factors that drives the cost of objective lenses and primary mirrors in optical systems is the fact that these optics have curved surfaces whose shape must comply with dimensional tolerances that are difficult to achieve.
Recently, it has become possible to employ a flat diffractive waveplate lens comprised of ultra-thin diffractive waveplate coatings on a thin, flat substrate, as the first optical element in some imaging systems, thus eliminating the need for a curved objective lens or curved primary mirror. See U.S. Non-Provisional patent application Ser. No. 15/189,551 filed Jun. 22, 2016, which claims the benefit of priority to U.S. Provisional Patent Application Ser. No. 62/182,975 filed Jun. 22, 2015, which are both incorporated by reference in their entirety.
This replacement has the potential to allow large cost and weight reductions for some applications. A critical advantage of using diffractive waveplate lenses is that they possess continuous structure that can be formed as micrometer thin film coatings on flat or curved shapes. Additionally, as opposed to other diffractive lenses such as Fresnel lenses, diffractive waveplate lenses are capable of near 100% diffraction efficiency in a broad band of wavelengths.
A fundamental property of diffractive waveplate lenses, as well as other types of diffractive elements, is that the angle through which electromagnetic radiation is deflected by such elements is highly dependent on the wavelength. In imaging systems, this produces chromatic aberrations, which can degrade the image quality.
For some applications, it has been shown that chromatic aberrations can be suppressed by means of a single corrector mirror. However, although the correction that is known to be possible with a single corrector mirror is sufficient for some applications such as laser communications, for which the optical signals are contained in a narrow spectral band, in other applications the degree of chromatic aberration correction is not sufficient.
In order to obtain the noted benefits of using a diffractive waveplate lens as the objective lens of an imaging system, it is necessary to identify a technique for reducing the chromatic aberrations of the diffractive waveplate lens used as the objective lens of an imaging system.
Thus, there is a need for a method and apparatus for further reducing the chromatic aberrations of diffractive waveplate lenses to an extent beyond the reduction achievable using prior art.